1. Field of the Invention
The present invention relates to a radiation curable composition for packaging printing, more specifically for high speed digital food packaging printing.
2. Description of the Related Art
Printing systems, such as offset and flexography, are being increasingly replaced for packaging applications by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing allowing last minute advertising changes on the packaging, and due to their enhanced reliability, allowing their incorporation into production lines. Radiation curable inkjet inks are particularly preferred because high quality images can be printed on non-absorbing ink-receivers, such as plastic packaging materials.
High reliability of inkjet printing on food packaging is not only required for reasons of productivity in an industrial environment, but also for reasons of food safety. The European Printing Ink Association (EuPIA) provides GMP guidelines for food packaging printing inks. In Europe most of the attention today is going to the Swiss legislation (“Ordinance on Materials and Articles in Contact with Food”, SR 817.023.21), promulgating a positive list of compounds. The US Food and Drug Administration (FDA) adheres to the no-migration principle and, therefore, does not impose specific guidelines on inks, except for direct food contact. A key figure in the allowable level of migration and/or set-off for ink compounds is 10 μg/6 dm2 (6 dm2 is the typical surface area of packaging material for 1 kg of food) per ink compound. This ratio of 10 μg/1 kg of food is also described as 10 ppb and is the rule-of-thumb for the allowable migration limit for an ink compound in the majority of legislations, but this limit can be higher, when substantiated by sufficient toxicological data.
Suitable UV curable inkjet inks for primary food packaging applications, often referred to as Low Migration (LM) inks, are exemplified by EP 2053101 A (AGFA), EP 2199273 A (AGFA) and EP 2161290 A (AGFA).
However, low migration UV curable inkjet inks as such do not exist. An ink formulation for printing on the outside of primary packaging can only contribute to safe food packaging. Also the packaging material and all conditions of the printing process should be monitored by migration testing. For example, phthalate plasticizers in packaging materials have attracted a lot of attention in the past and more recent reports involved the contamination of corn flakes by mineral oils from printing inks contained in recycled paper and carton.
From an engineering point of view, incorporating LED curing in manufacturing lines is considerably more convenient in comparison with classical mercury UV lamps and also reduces overall energy consumption. The evolution for curing UV curable inkjet inks from broad, high power mercury UV lamps to UV LEDs emitting in a narrow band at smaller UV light output has made low migration UV curable inkjet printing packaging solutions even more critical for printing reliability and food safety. The smaller UV light output of UV LEDs can be partly compensated by using a nitrogen blanket during curing. However in production lines, inertisation by using a nitrogen blanket complicates the design of the production line to such an extent that implementing digital printing into a production line is no longer economically feasible.
In addition, improper storage and transport conditions may also deteriorate the performance of UV curable LM inkjet inks. Not only the dispersion stability of colour pigments in the ink may be negatively impacted, but also curing speed may be reduced while migrateables can increase.
Hence, there is still a need for improved radiation curable inkjet inks which can be printed with high reliability, which can be cured by UV LEDs and which do not suffer under varying transport conditions of freezing temperatures and high temperatures.